1. Field of the Invention:
This invention relates generally to refrigeration systems and more particularly, it relates to a self-optimizing, capacity control system for inverter-driven centrifugal compressor based water chillers wherein adjustable inlet guide vanes and compressor speed are both automatically regulated in response to a continually updated "learned" chiller surge surface so as to realize minimum overall chiller energy consumption.
2. Description of the Prior Art:
In the art of inverter-driven chillers, it is generally known that the most efficient operating conditions are met when the chiller is operating at an operating point which has the combination of the lowest compressor speed and more open PRV position. However, the surge surface of the chiller restricts the degree to which the lowest speed and larger vane openings combination can be established. Thus, it should be clear that the objective of capacity control is to operate the chiller at the lowest, surge free, compressor speed possible, but yet still supplies the load and head requirements. In certain prior art attempts, approximations to the chiller surge surfaces are factory set by means of pre-stored surge surface equations contained in a capacity control unit. Since the surge surface for a particular chiller are not constant all the time and changes under certain conditions due to factors such as tube cleanliness, compressor aging, and drifts in transducers or sensors used for detecting PRV position, condenser pressure, and evaporator pressure, the use of constant pre-stored surge surface equations do not take into consideration these varying conditions.
It would therefore be desirable to provide a self-optimizing, capacity control system for inverter-driven centrifugal compressor based water chillers which continually adjusts itself for varying conditions affecting the surge surfaces so as to operate at minimum overall energy consumption.
U.S. Pat. No. 4,151,725 which issued on May 1, 1979 there is disclosed a capacity control system wherein the compressor head value was inferred from the difference between the temperature of liquid refrigerant leaving the condenser and the temperature of the saturated refrigerant leaving the evaporator. This temperature difference was utilized with the control system to regulate both the means for adjusting the position of the guide vanes and the means for driving the compressor. However, the assumed linearity of the function between the isentropic compressor head parameter .OMEGA. and the difference between the condensing and the evaporating refrigerant temperatures was not to be entirely accurate over the system operation, especially at light loads and at low heads.
In U.S. Pat. No. 4,282,719 which issued on Aug. 11, 1981, there is taught a capacity control system having a more accurate indication of compressor head which even at light loads and low heads, could be determined by a signal that is a function of the expression (P.sub.cd -P.sub.ev)/P.sub.ev. The numerator of this ratio is derived from the difference between the condenser absolute pressure P.sub.cd and the evaporator absolute pressure P.sub.ev. The denominator of this ratio signal is derived from the evaporator absolute pressure P.sub.ev.
In co-pending patent application Ser. No. 652,401 which was filed on Sept. 20, 1984, and which issued as U.S. Pat. No. 4,546,618 on Oct. 15, 1985, there is shown a capacity control system for inverter-driven centrifugal based water chillers which includes a microprocessor responsive to continual measurements of a PRV signal, compressor head signal and suction flow signal for regulating both the inlet guide vanes and compressor speed. In another embodiment of this co-pending application, the compressor head signal is replaced with an actual motor speed signal of the compressor. Consequently, the microprocessor in the second embodiment is responsive to the PRV signal, motor speed signal and suction flow signal for regulating both the inlet guide vanes and compressor speed.
However, the two above patents, and co-pending patent application, used pre-stored surge surface equations which are factory set. Thus, these systems did not take into account the varying conditions affecting the surge surfaces. The present invention represents an improvement over these two patents and co-pending patent application, which are all assigned to the same assignee as this application. This invention provides a self-optimizing, capacity control system for inverter-driven centrifugal compressor based water chillers wherein adjustable inlet guide vanes and compressor speed are both automatically regulated in response to a continually updated "learned" chiller surge surface so as to realize the minimum overall chiller energy consumption. This is accomplished by a microprocessor having a random-access memory which periodically identifies the location of the surge surface at the current operating point of the chiller and updates that surge surface information in the random-access memory during a "learning" mode. Then, the microprocessor causes the chiller to operate at a safety margin away from the surge surface during an "operating" mode for minimum energy consumption.
In particular, the present invention uses all of the same input signals to the control system block 50 in the '725 patent except that (1) the condenser temperature from the thermistor 56 is replaced by an absolute condenser pressure signal taken from the transducer 110 on line 111 of '719 patent and (2) the evaporater temperature from the thermistor 58 is replaced by an absolute evaporator pressure signal taken from the transducer 112 on line 113 of the '719 patent. Further, the amplifier 59 of the '725 patent is replaced by a functional generator 104 formed by the blocks 120 and 123 in FIG. 2 of the '719 patent. The substantial improvement of the present invention depends in part upon substituting the electronic circuitry of the control system 50 illustrated in FIG. 6A, 6B and 6C of the '725 patent by a microprocessor having a random-access memory for storing the updated surge surface information.